Optical arrays including, but not limited to, high resolution, linear arrays are frequently used in such applications as visual displays and recording printheads. These arrays are typically fabricated from compound semiconductor materials including certain III-V compounds. The GaAs/GaAsxP1−x combination is typical of such compounds. Light-emitting diode arrays as contemplated by the present invention comprise a plurality of small, closely spaced planar diffused islands of a first conductivity type, for example p-type, in a second conductivity type (n) material. The starting material for such arrays typically comprises a GaAs, n-type substrate on which is deposited an epitaxial layer of n-type GaAsxP1−x material. The individual light-emitting devices are formed by, for example, diffusing p-type material through windows formed in a dielectric layer grown on top of the epitaxial layer. Individual metal contacts such as aluminum are provided to the p-type material utilizing known evaporation techniques, the material being etched into individually addressable stripes with photolithographic processes which are well known and described in the literature.
Typically, the bottom face of the substrate (n-type) is a common connection for the array. By selectively energizing individual devices, an optical pattern can be developed for visual display purposes or for annotating or marking suitable photoreceptor material.
High density linear arrays such as might be used for recording printheads, are known to have a linear density in excess of 300 active devices per inch. In operation, many of these devices will be activated at the same time and the energy dissipated by the current flowing through multiple devices can lead to an increased temperature of the array. This increase in array temperature causes decreased output radiation with the decrease being approximately 1% per ° C.
As discussed previously, the back face or n side common contact is required to provide the return path for the activation current in a typical and previously described structure. In order to reduce thermal effects, it is known to mount such linear arrays on carriers that have good thermal characteristics. For example, aluminum blocks are commonly used as a mounting medium.
U.S. Pat. No. 5,192,958 which issued Mar. 9, 1993 to Chamitski discloses a metal carrier for an LED printhead in which liquid coolant is used to further decrease the temperature of the active devices.
U.S. Pat. No. 5,177,500 which issued Jan. 5, 1993 to Ng is also concerned with the thermal effects of operating a high density array, but this patent relates to the equalization of thermal effects between adjacent arrays.
One of the complications of utilizing a liquid cooled carrier, is that the thermal differential between the semiconductor substrate having relatively poor thermal conductivity and the cooled array carrier can lead to electrical connectivity problems between the two elements. The difference in thermal coefficients, under certain operating conditions, can cause a complete or partial breakdown of the electrical connection between the substrate and carrier. One of the common methods of mounting such arrays onto a carrier is to use conductive epoxies. These epoxies, under the above described operating conditions, are known to break down resulting in loss of optical output of the array. In other less catastrophic situations, the conductivity between the elements is decreased resulting in reduced optical output of the array.
The present invention provides a system and method for improving electrical and thermal characteristics of a high density LED array.